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The Bridge Problem

You get hired by a science industry. In the middle of the night, you wake up to get some cookies.
While you are downstairs in the laboratory kitchen, you see a button that says, "Do Not Press."
Guessed it yet? That's right, you pressed it.

The button opens up a few capsules that were holding zombies, and now they are after you. Quickly, you put your coat on and wake up the others, (who I will introduce in a minute.) When you get outside, you all start heading away from the lab, with the zombies chasing you out. Eventually, you run to a bridge, which is the only way to safety.

The bridge is too rickety that if more that two people at a time even step on it, It will collapse under your feet and you will fall into a huge gorge. Plus, it's so dark outside that the lantern you happened to salvage when evacuating the lab can only illuminate a small area of the bridge. This means that you have to hold the lantern or be right next to the person who is holding it.

Each of you take your own time to run across the bridge:

You

1 Minute

Assistant

2 Minutes

Janitor

5 Minutes

Elderly Professor

10 Minutes

Here, you can see them showing how fast they run, in minutes:

The professor calculates it will take 17 minutes for the zombies to catch up to you.

How many rounds will it take to get everyone across the bridge safely?

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